The correct answer is C. Novation of contract.
A novation is a contract that replaces an existing contract between two or more parties. In this case, the existing contract is between A and B, and the new contract is between B and C. The old debt of A is discharged, and a new debt from C to B is contracted.
Alteration of contract is a change to an existing contract. This can be done with the consent of all parties to the contract.
Rescission of contract is the cancellation of an existing contract. This can be done with the consent of all parties to the contract, or by a court order.
Charge in contract is a security interest that is created over property that is the subject of a contract. This can be done with the consent of the owner of the property, or by a court order.
In this case, the old contract between A and B is replaced by a new contract between B and C. This is a novation, not an alteration, rescission, or charge.